AT&T data/plan question

I had someone ask me if they have a 'Messaging & Data Unlimited' plan for feature phones, if it would continue to work with the SIM card in an iPhone. If AT&T will be able to tell it has changed to an iPhone, and if not, if any additional configuration is required. My instincts tell me that it wouldn't work, but I couldn't find any real information on the results of swamping the SIMs.

IF you have the wrong data plan then you may not access the internet. Eitherway AT&T will add the appropriate data plan to your account which is going to be the 200mb w/o visual voicemail or the unlimited plan.

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